With less metasurface element variety and complex wave control ability, acoustic coding metasurfaces (ACMs) show more competitive in the metamaterial application industry. However, hindered by their narrow-band modulation capability, the previous ACMs, especially the transmission type ACMs do not exploit broadband applicability. To address the frequency based coding capability, here we report broadband acoustic coding metasurfaces (BACMs) whose elements are designed by bottom-up topology optimization method.
Subsequently, utilizing the strategy, we designed the 1-bit coding sequence elements ‘0’ and ‘1’ (with out-of-phase responses) and 2-bit coding elements ‘00’, ‘01’, ‘10’ and ‘11’(with four phase states of 0°, 90°, 180°, and 270°). Optimized BACMs is helpful to improve the functions of a fixed coding sequence in the frequency dimension. In this paper, we present the broadband sound antenna that will deflect incident plane wave into wide variety of directions relying on frequencies. This idea provided inspiration for fabricating fast sector-scanning sound antenna. Combining 1-bit and 2-bit coding sequences, we also give the design strategy of the broadband fast scanning multiple-wave beam antenna. Furthermore, we also proved a pair of opposite functions, namely the broadband sound focusing and diffusion scattering characteristics which have potential applications in ultrasonic therapy, low-scattering targets design and noise control, and so on. Numerical simulation and experiments have proved the validity of the proposed coding elements and sequences.
